Autofocus systems in digital cameras mainly include contrast system and phase difference system. The contrast system involves moving lenses to achieve focus at the highest contrast. In digital cameras, autofocus is achieved by reading a part of an image captured by an image sensor, which eliminates the need for an optical system for autofocus.
The phase difference system employs what is called triangulation techniques for determining the distance from a subject to two different points by measuring an angular difference between the subject and the two points. For the phase difference system, the images of light passing through different regions of a lens, for example, the flux of light at the left and right sides of a lens is used. In the phase difference system, how long the lens needs to be moved to a focal position is achieved is determined by measuring the distance.
Image-plane phase difference autofocus performs autofocus with the phase difference system using an image sensor. The image sensor has condensing microlenses. The image sensor is further provided with a diaphragm for limiting light incident on the microlenses to give an image sensor for phase-difference autofocus (see, for example, Patent Literature 1).